#!/usr/bin/env perl

use warnings;
use strict;
use List::BinarySearch qw( binsearch );

my %hoa = (
    x => [qw(a b c d)],
    y => [qw(e f g)],
    z => [qw(h i c j)],
);

my $x = q(c);

for my $k ( keys %hoa ) {
    my @tmp = sort { $a cmp $b } @{ $hoa{$k} };
    my $idx = binsearch { $a cmp $b } $x, @tmp;
    if ( defined $idx ) {
        print qq($k => $tmp[$idx]\n);
    }
}

__END__

x => c
z => c