Eily beat me to it, but since I've already typed it up... ;-) B::Deparse to the rescue - the first two are localtime() - $x, while the third is localtime(-$x). The same is true if you're not using Time::Piece, which overrides localtime, but you also get a helpful warning. Interestingly, core localtime has a prototype, while Time::Piece::locatime doesn't.

$ perl -MTime::Piece -le 'print prototype("CORE::localtime"); print pr
+ototype("Time::Piece::localtime")//"undef"'
;$
undef

$ perl -MO=Deparse -e 'my $x = localtime; my $y = localtime - $x; prin
+t "$y\n";  '
Warning: Use of "localtime" without parentheses is ambiguous at -e lin
+e 1.
my $x = localtime;
my $y = localtime -$x;
print "$y\n";
-e syntax OK

$ perl -MO=Deparse -e 'use Time::Piece; my $x = localtime; my $y = loc
+altime() - $x; print "$y\n";  '
use Time::Piece;
my $x = main::localtime();
my $y = main::localtime() - $x;
print "$y\n";

$ perl -MO=Deparse -e 'use Time::Piece; my $x = localtime; my $y = (lo
+caltime) - $x; print "$y\n";  '
use Time::Piece;
my $x = main::localtime();
my $y = main::localtime() - $x;
print "$y\n";

$ perl -MO=Deparse -e 'use Time::Piece; my $x = localtime; my $y = loc
+altime - $x; print "$y\n";  '
use Time::Piece;
my $x = main::localtime();
my $y = main::localtime(-$x);
print "$y\n";
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