return (map{ /^$key/ and s/^$key//, $_ } grep( /^$key/, @ta))[1] ;
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Are you sure you want the comma operator inside the map? Try

map{ /^$key/ and s/^$key//; $_ } grep( /^$key/, @ta)
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The s/// inside the map evaluates to 1.

What I don't understand too is that when I replace the map expression with

map{ /^$key/ and s/^$key// and print "val=$_\n", $_ }
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it doesn't work verywell anymore, I only added a print statement ?

Same here. print "val=$_\n", $_ prints $_ twice, and map does return 1 (the result of print, if successful).

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}