That reminds me of the danger of passing a var to a function that modifies that var. It's particularly easy to screw up when dealing with built-in globals variables such as $1.

sub func {
   'b' =~ /(.)/;
   print(@_, "\n");
}

'a' =~ /(.)/;  func($1);    # Prints "b"
'a' =~ /(.)/;  func("$1");  # Prints "a"
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sub func {
   eval { die "bar\n" };
   print(@_);
}

eval { die "foo\n" };  func($@);    # Prints "bar"
eval { die "foo\n" };  func("$@");  # Prints "foo"
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(If you want to mess with your mind, try removing the second 'a' =~ /(.)/; from the first snippet.)