The basic problem with  $" = eval '"' . $delim . '"' is that someone can pass something like  `rm -rf /` as the delimiter argument to your script. A QND workaround would be to just reject anything over two characters in length for the delimiter argument... like so:

die "some trouble is brewing\n"  if length($ARGV[0]) > 2;

# Freely borrowed from ikegami
$delim = eval '"' . $ARGV[0] . '"';

print "Field1" . $delim . "Field2" . $delim . "Field3" . $/;
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Good luck