Perhaps because it produces the incorrect result and 8 more warning messages:
#!/usr/bin/env perl
use warnings;
use strict;
use List::Util qw/reduce/;
print reduce { $_[0] * $_[1] } 1..6;
__END__
Use of uninitialized value in multiplication (*) at x.pl line 7.
Use of uninitialized value in multiplication (*) at x.pl line 7.
Use of uninitialized value in multiplication (*) at x.pl line 7.
Use of uninitialized value in multiplication (*) at x.pl line 7.
Use of uninitialized value in multiplication (*) at x.pl line 7.
Use of uninitialized value in multiplication (*) at x.pl line 7.
Use of uninitialized value in multiplication (*) at x.pl line 7.
Use of uninitialized value in multiplication (*) at x.pl line 7.
Use of uninitialized value in multiplication (*) at x.pl line 7.
Use of uninitialized value in multiplication (*) at x.pl line 7.
0
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