Note, the new 'use warnings' pragma allows for various fine tuning of default and optional warnings -- including promoting any warning category into FATALs for a given lexical scope. So chipmunk's example can also be written as (assuming 5.6+):

for $invalue (qw/ 1 -10 abc /) {

    eval{
        use warnings FATAL=>qw/numeric/;
        $invalue + 0 
    };   ### Is Number?
    
    if ( $@ ) {
        print "$invalue is *not* a Number; Details: $@\n";
    } else {
        print  "$invalue appears to be a number.\n";
    }

}
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For details see the perllexwarn manpage, and the docs for the warning.pm module.