use strict;
use warnings;

my $path = 'file.htm';
my @check = ('href','src');
my ($handle, $text);

open($handle, $path);
$text = join('', <$handle>);
close($handle);

my $check = '(?:' . join('|', @check) . ')';
$text =~ s/($check=")([^"]+)/$1.relative($2)/ieg;

open($handle, ">$path");
print $handle $text;
close($handle);

sub relative {
    return $_[0] if length($_[0]) < 2;            # Ignore empty or / URLs
    return $_[0] if index($_[0],':') != -1;       # Ignore email links
    return $_[0] if substr($_[0],0,1) eq '/';     # Ignore absolute URLs
    if (substr($_[0],0,1) eq '.') {
        return $_[0] if substr($_[0],1,1) eq '.'; # Ignore URLs already ../
        return '.'.$_[0];                         # URLs ./ change to ../
    }
    return '../'.$_[0];                           # Rest changes to ../
}