my @extList;
push @extList, $_ for @ARGV;
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How is that different from  my @extList = @ARGV; (other than being less efficient)?

I'd do

use Getopt::long;
my @extList;
my $root;
GetOptions(
  'ext=s' => \@extList,
  'root|dir=s' => \$root,
);
@extList = @extList
  ? map { split /,/ } @extList; # in case of --ext foo,bar
  : qw( pl pm );
$root ||= '.';
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