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# untested
for(@str) {
   /^(?:(\w{2}_[a-z0-9]+)|([a-z0-9]+)_)/i and print $1 || $2, "\n";
}

for(@str) {
    /^(\w{2}_[a-z\d]+|[a-z\d]+(?=_))/i and print $1, $/;
}