pack q{NNa*}, m{(\d+)_(\d+)}, $_
Note that packing with an 'N' specifier fails if either of the numeric fields exceeds 4294967295. An approach using sprintf (as mentioned in johngg's reply) can get around this. 
>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

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In reply to Re^2: Sort problems by AnomalousMonk
in thread Sort problems by erez_ez

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