Maybe this will help your understanding:

#!/usr/bin/perl
use strict;
use warnings;
my %hash=(1=>10,2=>20,3=>30,4=>40,5=>50);


my $max=0;
my @out=map{
    $_=$hash{$_};
    if($_>$max){$max=$_} else { ()}
#    $_;
}keys %hash;

print "Max value was $max\n";
use Data::Dumper; print Dumper \@out;

# prints
Max value was 50
$VAR1 = [
          40,
          50
        ];
[download]

Note that the result of a block is the result of the last statement of a block. In case of an if-clause that might be the false value, in string context the empty string ''

PS: What for you is strangeness is for most of us a script working as it should. If you don't tell us what you would expect as output we can't tell you where your expectation is wrong