Two problems with the substitution you tried:

$temp = s|_|.|;
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First, you need a tilde: $temp =~ s///

Second, you need the /g operator so that more than just the first "_" is replaced.

sub sk{ return unless $in = shift;
        $in =~ s!(.)$!!; print $1; sk($in)}
sk("gro.alubaf@yehaf");
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