$ 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