>perl -wMstrict -le
"my @list = qw{ a1_2 a1_1 a10_10 a2_10 a2_1 a2_2 a10_1 a10_2 a1_10 };
 use constant WIDTH => 20;
 my @sorted =
    map { substr $_, WIDTH * 2 }
    sort
    map { sprintf '%0*4$d%0*4$d%s', m{(\d+)_(\d+)}, $_, WIDTH }
    @list;
 print for @sorted;
"
a1_1
a1_2
a1_10
a2_1
a2_2
a2_10
a10_1
a10_2
a10_10