#!/usr/bin/perl

my %hash=(
        'a',    '2',
        'b',    '5',
        'a+b',  '7',
        ); 

my $string='a+b';
my $key;

foreach $key (keys %hash){
  if($string=~/$key/){
    print "I found a match: \"$key\"\n";
  }else{
    print "There was no match for \"$key\"\n";
  }
}