Try this:

use List::Util qw[ reduce ];

$in = 'poe1 poe2 wap1 wap2';;

$out = reduce{ 
    $a .= $a=~/wap/ && $b=~/^wap(\d+)/ ? ",$1" : " $b" 
} split ' ', $in;;

print $out;;
poe1 poe2 wap1,2

$in = 'poe1 poe2 wap1 wap2 wap13 wap47';;

$out = reduce{ 
    $a .= $a=~/wap/ && $b=~/^wap(\d+)/ ? ",$1" : " $b" 
} split ' ', $in;;

print $out;;
poe1 poe2 wap1,2,13,47
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