If you want to be certain that your script only searches a local /etc/passwd file, you'll have to do it yourself. On the bright side, it's not hard to do that:

open FH, '/etc/passwd' or die "Can't open /etc/passwd, $!";
my @matches = grep { chomp; /thing_to_match/ } <FH>;
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or you can iterate through the file & split out each line like so:

while (<FH>) {
   chomp;
   my ($name, $passwd, $uid, $gid, $quota, $comment,
       $gcos, $dir, $shell) = split /:/;
}
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The following code mimics getpwnam:

sub my_getpwnam {
   my $name_m = shift;
   open FH, '/etc/passwd' or die "Can't open /etc/passwd, $!";
   
   my @match = grep { chomp; /^$name_m:/ } <FH>;
   if (!@match) { return (); }  # return empty list if not found
   return split /:/, $match[0];
}
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If you really don't want to do it yourself, tilly's Text::xSV module could be your answer.

Update: Added a missing brace & stuck in an error check