When i try to sort the input data, i am not getting the wrong results

#!/usr/bin/env perl
my (@months) = qw(Mar13 Jun13 Jul13 Apr13 );

my @mon = map uc, qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 
+);
my $mon;
@{$mon}{@mon} = ( 0 .. $#mon );

sub by_month {
    my ($a_mon, $a_yy) = ($a =~ /(...)(..)/);
    my ($b_mon, $b_yy) = ($b =~ /(...)(..)/);
    $a_yy <=> $b_yy or $mon->{$a_mon} <=> $mon->{$b_mon};
}

@months = ( sort by_month @months );

print "@months\n";
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output

Mar13 Jun13 Jul13 Apr13
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please help me on this</P<