I've been using the following script, placed at ~/bin/whichpm.

#!/usr/bin/perl
use File::Spec::Functions qw(:ALL);

$ARGV[0] or die "usage: $0 Module::Name\n";

my $subpath = catfile(split '::', $ARGV[0]).".pm";

for(@INC) {
    my $fullpath = catfile($_, $subpath);
    print("$fullpath\n"), exit if -f $fullpath;
}

die +(splitdir $0)[-1] . ": no such module $subpath\n";
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Makeshifts last the longest.