Or this:

use warnings;
use strict;

my @p = qw[PA5098  PA5100  PA5092  PA3175];

for my $i ( 0 .. $#p ) {
    last if $i == $#p;
    for my $j ( 0 .. $#p ) {
        print join( " " => ( $p[$i], $p[$j], $j - $i ) ), $/ unless $i
+ >= $j;
    }
}
[download]

Output

PA5098 PA5100 1
PA5098 PA5092 2
PA5098 PA3175 3
PA5100 PA5092 1
PA5100 PA3175 2
PA5092 PA3175 1
[download]

I kind of like AnomalousMonk solution, because it has no help from ifs and unless. McA solution is also cool for me, but I don't like C-for loop in perl! :)