If it were me, I'd use the list of valid collections that you already have (assuming I understand your data correctly). Something like:

# Note: Still has underscsores:
my @collections = map { $_ =~ s/\.txt//; $_ }
                  grep {/^\p{uppercase}/}
                  @collections_list;

# $collection guaranteed to match one of your file names.
my $collection_raw = $cgi->param('collection');
my ($collection)   = grep { $_ eq $collection_raw } @collections;
$collection      //= 'Deafult';
# If your file names might have <>", you can still do this:
$collection        = encode_entities($collection);

# Without underscores
my @collection_labels = map { $_ =~ s/_/ /g; $_ } @collections;
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Generally, it is safest to do something like this before using $collections for anything.