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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In reply to Re^2: Name "main::a" used only once: possible typo by toolic
in thread Name "main::a" used only once: possible typo by szabgab

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