I'm not sure I understand your question. That said, I'd recommend you look into using split for what you appear to be doing. Here's a quick example:

use Data::Dumper;

my $restofline = "One fish two fish red fish blue fish";
print Dumper( [ split /\s*fish\s*/, $restofline ] );
__END__
$VAR1 = [ 
          'One',
          'two',
          'red',
          'blue'
        ];
[download]

If for some reason, you really do have to process the string incrementally, maybe index is a good solution (provided your keyword really is static).

If you really do what to use a regex, I wonder if a non-greedy match is what you're after. That is, "$restofline =~ m/(.*?)fish(.*)/".