use POSIX 'tmpnam';

my $cmd = "cat /dev/null";
my $trace_file = tmpnam;

system "strace -f -e trace=open -o $trace_file $cmd";

my %files = ();

open TRACE, "<$trace_file";
while (<TRACE>)
{
    /^open\("(.*)", O_/ and $files{$1}++;
}
close TRACE;
unlink $trace_file;

print join "\n", sort keys %files;