$ perl -e '
> use strict;
> use warnings;
> my @in = split /\n/,
> "a 123
> a 125
> b 127
> a 129
> a 130
> b 131
> a 132
> b 133";
>
> my @a_buff;
> for (@in) {
>   push @a_buff, $_ and next if /a/;
>   my $aa = shift @a_buff; # avoid to use the $a special variable
>   my $diff = $2 - $1 if "$aa - $_" =~ /(\d+)[^\d]*(\d+)/;
>   print "$aa ; $_ => $diff\n";
> }
> '
a 123 ; b 127 => 4
a 125 ; b 131 => 6
a 129 ; b 133 => 4

##</code><code>##

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

##</code><code>##

$ perl iter_pairs.pl
a 123 ; b 127 => 4
a 125 ; b 131 => 6
a 129 ; b 133 => 4

##</code><code>##


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
...