#sub change_ext_if_file {
    #foreach my $file ($_) {
     # if (($file eq -f) && ($file =~ /.pdf$/)) {
           # $file =~ s/.pdf/.doc/;

    #Above, better written as below, perhaps?
sub change_ext_if_file {
    my @files = $_;
    foreach my $file if ((@files eq -f) && (@files =~ /\.pdf$/)) {
            $file =~ s/pdf/doc/;
    }
    }