/usr/bin/perl -we '
   use strict;
   sub fn {
      my ($str) = @_; print "<@_>\n";
      $str =~ s/x//;  print "<@_>\n";
   }
   $_ = "x";
   s/(x)/fn $1/e;
'
<x>
Use of uninitialized value $_[0] in join or string at -e line 5.
<>