Without using File::Spec, you could do (replacing the values as needed):

#!/perl/bin/perl -w

use strict;
use Data::Dumper;

my $dir       = 'C:/WINNT';
my $searchext = 'dll';

opendir(DIR, $dir) || die qq(Could not open directory "$dir"\n);

my @files = grep { /\.${searchext}$/ } readdir(DIR);

print Dumper(\@files);
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Update:  If you need to check for these files also in the subdirectories (or multiple other directories), my knee-jerk reaction is to use File::Find, which works very well for that purpose.