If you are using multiple files in your script and you assume that "use strict" from the main script will automatically affect all the other scripts, you will run afoul of this scoping issue. The scripts below will exhibit different behavior with respect to globals when strict and my are used than when strict and my are not used. If we really want to use $z in our main scope as a global, we should either pass a reference to it in our sub call (and modify it in place), or set its value based on the return value of our sub call. Or we could not use strict, but we should use strict and figure out a better way to handle our data than indiscriminate use of globals.
#this is scope1.pl
use strict; #comment out
my $z = 0; #comment out, we could also leave this
# undef, but that generates a bunch of warnings
for my $x (1..5)
my $y = multiply( $x );
print "$x * 3 : $y : $z\n";
#this is scope2.pl
$this = shift;
$z = $this * 3;
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