use strict;
use warnings;

my %hash;
m/(\d+),(\d+)/ and push(@{$hash{$1}}, $2) for <DATA>;

for (sort keys %hash) {
    print "$_ => ";
    print "$_ " for sort @{$hash{$_}};
    print "\n";
}

__DATA__
2,6
2,3
1,2
2,5
1,3
1,4