$ cat unexpected.pl
#!env perl
use strict;
use warnings;

my $S4;

sub X {
    print "<$S4>\n";
    return $S4;
}

for $S4 (1 .. 2) {
    print "A: <$S4> <", X(), ">\n";
}

$ perl unexpected.pl
Use of uninitialized value $S4 in concatenation (.) or string at unexpected.pl line 8.
<>
Use of uninitialized value in print at unexpected.pl line 13.
A: <1> <>
Use of uninitialized value $S4 in concatenation (.) or string at unexpected.pl line 8.
<>
Use of uninitialized value in print at unexpected.pl line 13.
A: <2> <>