use strict;
use warnings;


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

sub create_iter {
	my @a_buff;
	return sub {
		while (<DATA>){
			chomp;
			push @a_buff, $_ and next if  /a/;
			return shift @a_buff, $_;
		}
	}
}
__DATA__
a 123
a 125
b 127
a 129
a 130
b 131
a 132
b 133