A quick test made at the command line, copying and pasting BrowserUk's test data (thanks to him), and separating the data into two parts: 
$ perl -e '
> use strict;
> use warnings;
> my $in =
> "a 123
> a 125
> b 127
> a 129
> a 130
> b 131
> a 132
> b 133";
>
> my @a = grep /a/, split /\n/, $in;
> print shift @a, " => $_\n" for grep /b/, split /\n/, $in;
> '
a 123 => b 127
a 125 => b 131
a 129 => b 133

[download]
Just 2 lines of actual code, but not golfing either in the slightest manner, I think it is fairly clear and straight forward.

Separating the data into two lots has an additional advantage if the input data is not ordered because sorting is just trivial: 

$ perl -e '
> use strict;
> use warnings;
> my $in =
> "a 123
> a 125
> b 131
> a 130
> b 127
> a 129
> a 132
> b 133";
>
> my @a = sort grep /a/, split /\n/, $in;
> print shift @a, " => $_\n" for sort grep /b/, split /\n/, $in;
> '
a 123 => b 127
a 125 => b 131
a 129 => b 133

[download]

Update: I forgot to include the calculation of the elapsed time, but this is so simple that it is left as an exercise to the OP.

Je suis Charlie.

In reply to Re: processing a list of events by Laurent_R
in thread processing a list of events by BluePerlDev

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