use Data::Dumper;

my %g_modules;

Traverse( $_ ) for ( @INC );

print Dumper \%g_modules;

sub Traverse {
        my $dir = shift;
        opendir my $dh, $dir or die "$!: $dir\n";
        for my $file ( grep !/^\./, readdir $dh ) {
                my $path = "$dir/$file";
                if ( -d $path ) {
                        Traverse ( $path );
                }
                elsif ( $file =~ /\.pm$/ ) {
                        $g_modules{ $file } = 1;
                }
        }
        closedir $dh;
}