If the string to be found is always a prefix, you can use a trie.

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };
use experimental qw( signatures );

sub trie_exists($trie, $key) {
    my $ref = $trie;
    for my $char (split //, $key) {
        return unless exists $ref->{$char};

        $ref = $ref->{$char};
    }
    return 1
}

my %prices = (pizza    => 12,
              coke     => 1.25,
              sandwich => 3);

my %trie;
for my $key (keys %prices) {
    my @chars = split //, $key;
    my $ref = \%trie;
    for my $char (@chars[0 .. $#chars - 1]) {
        $ref->{$char} //= {};
        $ref = $ref->{$char};
    }
    $ref->{ $chars[-1] } = $prices{$key};
}

if (trie_exists(\%trie, 'cok')) {
    say "found the preifx 'cok' in the hash";
}

die 'Full key' unless trie_exists(\%trie, 'pizza');
die 'Non prefix' if trie_exists(\%trie, 'andwich');
[download]

There are also several trie modules on CPAN, but I haven't tried any of them, so can't make a recommendation.