In the substitution operator the right hand side is evaluated in scalar context and thus localtime returns the formatted date as a string as per documentation:

use strict;
use warnings;

my @x = localtime 1503403724;
print "@x\n";

my $x = localtime 1503403724;
print "$x\n";

my $z = "ccc";
$z =~ s/ccc/wantarray()?"list":"scalar"/e;
print "$z\n";
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