Something like this?:

#! perl -slw
use strict;

chomp( my @events = <DATA> );

for( my( $lastA, $lastB ) = ( 0, 0 ); $lastA < @events and $lastB < @e
+vents; ++$lastA, ++$lastB ) {
    ++$lastA until $events[ $lastA ] =~ m[^a\s(\d+)];
    my $aTime = $1;
    ++$lastB until $events[ $lastB ] =~ m[^b\s(\d+)];
    my $bTime = $1;
    print "$events[ $lastB ] - $events[ $lastA ] := ", $bTime - $aTime
+;
}

__DATA__
a 123
a 125
b 127
a 129
a 130
b 131
a 132
b 133
[download]

Produces:

C:\test>junk
b 127 - a 123 := 4
b 131 - a 125 := 6
b 133 - a 129 := 4
[download]