my $input = 'foo+bar+baz';
my $i = 1;
my $string = join '&', map { 'keyword' . $i++ . '=' . $_ } split /\+/,
+ $input;
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This might be the first time you see map. It's not as hard as it looks: map BLOCK LIST evaluates BLOCK for every item of the given LIST, aliassing $_ to that item.
In other words: map creates a list out of a list.

So this is what happens: $input gets split, and the individual items are sent through map, which prefixes "keyword#=", increasing the number by one. Then, join joins them using supergl^W'&'. The result is put in $string.

2;0 juerd@ouranos:~$ perl -e'undef christmas'
Segmentation fault
2;139 juerd@ouranos:~$
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