How can I skip lines when slurping a file in Perl

The basic idea of inserting, changing, or deleting a line from a text
file involves reading and printing the file to the point you want to
make the change, making the change, then reading and printing the rest
of the file. Perl doesn't provide random access to lines (especially
since the record input separator, $/, is mutable), although modules
such as Tie::File can fake it.

A Perl program to do these tasks takes the basic form of opening a
file, printing its lines, then closing the file:

open my $in,  '<',  $file      or die "Can't read old file: $!";
open my $out, '>', "$file.new" or die "Can't write new file: $!";

while( <$in> )
       {
       print $out $_;
       }

   close $out;

Within that basic form, add the parts that you need to insert, change,
or delete lines.

To prepend lines to the beginning, print those lines before you enter
the loop that prints the existing lines.

open my $in,  '<',  $file      or die "Can't read old file: $!";
open my $out, '>', "$file.new" or die "Can't write new file: $!";

print $out "# Add this line to the top\n"; # <--- HERE'S THE MAGIC

while( <$in> )
       {
       print $out $_;
       }

   close $out;

To change existing lines, insert the code to modify the lines inside
the while loop. In this case, the code finds all lowercased versions
of "perl" and uppercases them. The happens for every line, so be sure
that you're supposed to do that on every line!

open my $in,  '<',  $file      or die "Can't read old file: $!";
open my $out, '>', "$file.new" or die "Can't write new file: $!";

print $out "# Add this line to the top\n";

while( <$in> )
       {
       s/\b(perl)\b/Perl/g;
       print $out $_;
       }

   close $out;

To change only a particular line, the input line number, $., is
useful. First read and print the lines up to the one you want to
change. Next, read the single line you want to change, change it, and
print it. After that, read the rest of the lines and print those:

while( <$in> )   # print the lines before the change
       {
       print $out $_;
       last if $. == 4; # line number before change
       }

my $line = <$in>;
$line =~ s/\b(perl)\b/Perl/g;
print $out $line;

while( <$in> )   # print the rest of the lines
       {
       print $out $_;
       }

To skip lines, use the looping controls. The next in this example
skips comment lines, and the last stops all processing once it
encounters either END or DATA.

while( <$in> )
       {
       next if /^\s+#/;             # skip comment lines
       last if /^__(END|DATA)__$/;  # stop at end of code marker
       print $out $_;
       }

Do the same sort of thing to delete a particular line by using next to
skip the lines you don't want to show up in the output. This example
skips every fifth line:

while( <$in> )
       {
       next unless $. % 5;
       print $out $_;
       }

If, for some odd reason, you really want to see the whole file at once
rather than processing line by line, you can slurp it in (as long as
you can fit the whole thing in memory!):

open my $in,  '<',  $file      or die "Can't read old file: $!"
open my $out, '>', "$file.new" or die "Can't write new file: $!";

my @lines = do { local $/; <$in> }; # slurp!

       # do your magic here

print $out @lines;

Modules such as File::Slurp and Tie::File can help with that too. If
you can, however, avoid reading the entire file at once. Perl won't
give that memory back to the operating system until the process
finishes.

You can also use Perl one-liners to modify a file in-place. The
following changes all 'Fred' to 'Barney' in inFile.txt, overwriting
the file with the new contents. With the -p switch, Perl wraps a while
loop around the code you specify with -e, and -i turns on in-place
editing. The current line is in $. With -p, Perl automatically prints
the value of $ at the end of the loop. See perlrun for more details.

perl -pi -e 's/Fred/Barney/' inFile.txt

To make a backup of inFile.txt, give -i a file extension to add:

perl -pi.bak -e 's/Fred/Barney/' inFile.txt

To change only the fifth line, you can add a test checking $., the
input line number, then only perform the operation when the test
passes:

perl -pi -e 's/Fred/Barney/ if $. == 5' inFile.txt

To add lines before a certain line, you can add a line (or lines!)
before Perl prints $_:

perl -pi -e 'print "Put before third line\n" if $. == 3' inFile.txt

You can even add a line to the beginning of a file, since the current
line prints at the end of the loop:

perl -pi -e 'print "Put before first line\n" if $. == 1' inFile.txt

To insert a line after one already in the file, use the -n switch.
It's just like -p except that it doesn't print $_ at the end of the
loop, so you have to do that yourself. In this case, print $_ first,
then print the line that you want to add.

perl -ni -e 'print; print "Put after fifth line\n" if $. == 5' inFile.txt

To delete lines, only print the ones that you want.

perl -ni -e 'print unless /d/' inFile.txt

       ... or ...

perl -pi -e 'next unless /d/' inFile.txt