I will probably write this using List::Util like so:

#!/usr/bin/perl
use warnings;
use strict;
use List::Util qw(first);

my $var = 'apple';

my $h_ref = {
    banana     => 1,
    plum       => 1,
    apple      => 1,
    strawberry => 1,
    pea        => 1,
};

my $foo = first { $_ eq $var } keys %{$h_ref};
 
defined $foo ? print "yes" : print "No";
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If your $var exist, it prints "Yes", if not you get a "No"