use strict;
use warnings;

# Put your subs here before any file-scope variables.
# That makes it clearer that each sub does not depend on
# anything other than the parameters passed to it.

sub ownertbl {
  my ( $sortby, $ascdesc, $letter ) = @_;    # added leading "my"
  # ...
}

# put your other subs here...

# Now define your lexical variables after the subs...
my ($ownsel, $sortby, $letter, $ascdesc, $t);

if ( $cgi->param ) {
  if ( defined ( $cgi->param( 'sortby' ) ) ) {
     $sortby = $_;    # this looks wrong to me
  } else {
     $sortby = "name";
  }
  if (defined ( $cgi->param( 'ascdesc' ) ) ) {
     $ascdesc = $_;   # ditto
  } else {
     $ascdesc =  "desc";
  }
  if (defined ( $cgi->param( 'letter' ) ) ) {
     $letter = $_;    # ditto
  } else {
     $letter = " ";
  }
  ownertbl( $sortby, $ascdesc, $letter );  # now these three variables are properly passed to the sub
} else {
  homepage();
}