I got a warning message which I can't understand. 
$ perl ./a
Useless use of numeric gt (>) in void context at /work/tmp/a line 4.

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where script './a' goes: 
#!/usr/bin/perl
use warnings;
my $flag=  ((-M '/tmp/a') > (-M '/tmp/b'));          #no warnings
my $flag2= $flag and ((-M '/tmp/c') > (-M '/tmp/d')); #warning at here

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My guess is that the Perl optimizer considers "-M '/tmp/c'" as just a boolean expression by itself, not a part of scalar comparison.
In real case, I want to write a condition which tests 1) existence of some file $file1. 2) when $file1 exists, test if $file1 is older than the other file($file2). 
if(-f $file1 and  ((-M _) > (-M $file2))){ ...

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raise a waring message. How should I write in this case?
My perl version is 'v5.8.4 built for i386-linux-thread-multi'.

In reply to Warning with file test(-M) by na

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