robby_dobby already pointed out the actual mistake: you need to pass fnd_max() a reference to the hash, since that's what fnd_max() is expecting. See how I pass my hashes to set_column widths().

Now beyond that: First, I'd use a more descriptive subroutine name, like "max_value_of_hash", and drop the prototype. Prototypes are advanced juju and shouldn't be used most of the time. Second, if you want to get the largest value from a hash, you don't need to access the keys at all. Here are some examples, starting with the simplest and wordiest:

#!/usr/bin/env perl
use 5.010; use strict; use warnings;

# newbie but clean version
sub max_value_of_hash {
    my $h = shift;
    my $max = 0;
    for my $v (values %$h){
        if ($v > $max){
            $max = $v;      # keep setting $max to larger value
        }
    }
    return $max;
}

# more perlish and elegant version
sub max_value_of_hash2 {
    my $h = shift;
    my $max = 0;
    $_ > $max ? $max = $_ : undef for values %$h;
    return $max;
}

# let a module do it
sub max_value_of_hash3 {
    use List::Util qw(max);
    return max values %{$_[0]};
}

my %hash = ( a => 1, b => 2, c => 5, d => 3 );
say max_value_of_hash( \%hash);
say max_value_of_hash2(\%hash);
say max_value_of_hash3(\%hash);
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