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#!/usr/bin/perl -w
sub trace { print join(' ', map { defined($_) ? $_ : '#undef#' } @_), 
+"\n"; }
...

$val=7.3;
trace('Final value is', $val);

roboticus@swill $ ./foo.pl
Before first chunk, value is #undef#
Before second chunk, value is foobar
Final value is 7.3