#! perl -slw
use strict;

my $pair = 'A25 A15';

## Assumes the 6 of 125 you need are the 3rd, 33rd, 50th, 75th, 100th, and 123rd,

my $tmplNeeded = join ' ', <<"EOT";
    x[($pair)2]     # skip 2
    $pair           # grab the 3rd
    x[($pair)29]    # skip 29
    $pair           # grab the 33rd
    x[($pair)16]    # skip 16
    $pair           # Grab the 50th
    x[($pair)24]    # ...
    $pair
    x[($pair)24]
    $pair
    x[($pair)22]
    $pair
EOT

open FILE, '<', ...;

while( <FILE> ) {
    my @required = unpack $tmplNeeded, $_;
    ## $required[ 0 ] is the 3rd field
    ## $required[ 1 ] is the 33rd field etc.
}