In the spirit of TIMTOWTDI, here's yet another way. A kind of slurp into split into sort, all wrapped in a printing subroutine.

use strict;
use warnings;

{
    local $/;
    printInTwos(
       sort {$a <=> $b}
       split m{(?:,|\s+)}, <DATA>);   
}

sub printInTwos
{
    print(shift, q{,}, shift, qq{\n}) while @_;
}

__END__
10,20
30,10
20,70
70,80
40,90
90,100
50,50

[download]

The output is

10,10
20,20
30,40
50,50
70,70
80,90
90,100

[download]

Cheers,

JohnGG

Update: I've just re-read the OP and realised I totaly misunderstood the requirement. Please ignore this post.

In reply to Re: Sorting Issue by johngg
in thread Sorting Issue by farhan

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