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 Jul11 = 1107
 Jun11 = 1106
 Aug13 = 1308
 May14 = 1405
 etc...

sub sortKeys {
    my ( $l_mo, $l_yr ) = $a =~ /(\D+)(\d+)/;
...
    my $right = $r_yr . $months->{$r_mo};
    return ( $left cmp $right );
}

sub sortKeys {
    ( my $left  = $a ) =~ s/(\D+)(\d+)/$2 . $months->{$1}/e;
    ( my $right = $b ) =~ s/(\D+)(\d+)/$2 . $months->{$1}/e;
    return ( $left cmp $right );
}

#!/usr/bin/perl 

...
Key - May14  and b value - 2