sub writeUnicode($$)
 {my ($f, $s) = @_;
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Assigns parameter 1 to the subroutine from @_ to $f and parameter 2 to $s. One could also write my $f = $_[0]; my $s = $_1;

  if ($f =~ /\A(.+[\\\/])/)
   {my $d = $1;
    makePath($d);
   }
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Extracts the path component of the file name and places it in variable $d so that we can make a directory for the output file. If you know that the output directory exists, then there is no need for this code. The assignment to $d is somewhat verbose, one could use makePath($1) to use the first expression captured by the regular expression in the if statement directly. The regular expression captures the text up to the last \ or / in the file name and uses that as the path component.

  say {$F} $s;
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Writes the contents of $s to the file whose handle is in $F. The file is automatically closed at the end of the block containing my $F. This statement is an alternative to $F->say($s);