I would most likely write that regex like this:

#!/usr/bin/perl -w
use strict;

foreach (<DATA>) {
    s/^i ([^\n]+)$/makeImageTag($1)/e; #see Update
}

sub makeImageTag {
    my $foo = shift || 'no_image.png';
    print "$foo\n";
}

__DATA__
i howdy.gif
i foo.jpg
i bar.png
i blah.jpg
[download]

Here you have perl handle the newlines

Can someone explain what the difference is between \1 and $1? Obviously I can't use the former when there's an expression to be evaluated...

from perlre:

$pattern =~ s/(\W)/\\\1/g;

This is grandfathered for the RHS of a substitute to avoid shocking the sed addicts, but it's a dirty habit to get into. That's because in PerlThink, the righthand side of a s/// is a double-quoted string. \1 in the usual double-quoted string means a control-A. The customary Unix meaning of \1 is kludged in for s///. However, if you get into the habit of doing that, you get yourself into trouble if you then add an /e modifier.

also you can ponder this from perlre s/(\d+)/\1000/;

Update: Jeez, I should've rewritten the whole regex. Absolutly no need for the negated char class  ([^\n]+) should be just (.+) since I thoughtfully pointed out the <> takes care of newlines

grep

grep> cd /pub grep> more beer