Changed my answer. This one:

• Ensures end-to-end match
• Allows case-insensitive (ComboBox, user might enter wrong case?)
• Breaks the steps up so it's easier to read

$ cat ./arrayreducto.pl
#!/usr/bin/perl
use strict;
use warnings;

my $old_combo = "USA,Canada,Australia";
print "Old Combobox:  $old_combo\n";

my $str = 'Canada';
print "Selected:  $str\n";

my @old_combolist = split /\,/, $old_combo;
my $str_regex = quotemeta $str;
my @new_combolist = ();

foreach my $old_comboitem (@old_combolist) {
    # Must match end to end but user may have entered in wrong case
    if ($old_comboitem !~ /^$str_regex$/i) {
        push @new_combolist, $old_comboitem;
    }
}

my $new_combo = join ',', @new_combolist;
print "New Combobox:  $new_combo\n";

$ perl ./arrayreducto.pl
Old Combobox:  USA,Canada,Australia
Selected:  Canada
New Combobox:  USA,Australia
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