This is one of the cases where MO::Deparse can help, by showing how perl interpretes your code:

perl -MO=Deparse -le 'use Time::Piece; $x = localtime; $y = localtime 
+- $x; print $y;'

BEGIN { $/ = "\n"; $\ = "\n"; }
use Time::Piece;
$x = main::localtime();
$y = main::localtime(-$x);
print $y;
-e syntax OK
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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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-MO=Deparse,-p is even clearer

Hi, ... and even without deparsing, you can see that under Time::Piece what localtime returns is different to what it assigns to a variable in scalar context:

$ perl -MTime::Piece -E 'say localtime; $x = localtime; say $x'
4351751111833380
Wed Dec  5 17:05:43 2018
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Hope this helps!