use strict;
use warnings;
use feature "state";

while (my ($aa, $bb) = iter()) {
	last unless defined $bb;
	my $diff = $2 - $1 if "$aa $bb" =~ /(\d+)[^\d]*(\d+)/;
	print "$aa ; $bb => $diff\n";
}

sub iter {
	state @a_buff;
	while (<DATA>){
		chomp;
		push @a_buff, $_ and next if  /a/;
		return shift @a_buff, $_;
	}
}
__DATA__
a 123
...