Negatives are tricky, but I think this will help you.

#!/usr/bin/perl

use strict;
use warnings;
use feature 'say';

my $return = find_string(
   file           => "/path/to/file",
   positive_match => 1,
   string         => "abc"
); 

say "did match? $return";

$return = find_string(
   file           => "/path/to/file",
   negative_match => 0,
   string         => "abc"
); 

say "did not match? $return";

sub find_string
{
   my %params = @_;
   my $found = 0;

# assumes that file is not tainted.
   # open my $fh, '<', $param{file} or die "Cannot opne $param{file}, 
+$!";
   
   while (<DATA>) { $found++ if m/\Q$params{string}/ }
   
   return 1 if ( $params{positive_match} && $found > 0 );
   return 0 if ( $params{positive_match} && $found == 0 );
   return 1 if ( $params{negative_match} && $found == 0 );
   return 0 if ( $params{negative_match} && $found > 0 );
}

__DATA__
abc
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