$ perl u.pl
Use of uninitialized value $file in concatenation (.) or string at u.pl line 16.
Use of uninitialized value $file in concatenation (.) or string at u.pl line 21.
OK, missing
OK, missing
OK, OK
$ cat u.pl
use strict;
use warnings;

my $f1;
my $f2='';
my $f3='README';

my $dir='./xyz';

print chk($dir, $f1), ", ", chk2($dir,$f1),"\n";
print chk($dir, $f2), ", ", chk2($dir,$f2),"\n";
print chk($dir, $f3), ", ", chk2($dir,$f3),"\n";

sub chk {
    my ($dir, $file) = @_;
    return -e "$dir/$file" ? "OK" : "missing";
}

sub chk2 {
    my ($dir, $file) = @_;
    return -f "$dir/$file" ? "OK" : "missing";
}