use constant DEFAULT_FILE => "/etc/filesystems";
my $file = shift || DEFAULT_FILE; # e.g. to take file from command line or use the default
find_pattern($file, $pattern);

sub find_pattern {
  my ($file, $pattern) = @_;
  # your code here
}

##</code><code>##

...
die "oh no, it's not there!" unless find_pattern($file, $pattern);

sub find_pattern {
  my ($file, $pattern) = @_;
  return `grep $pattern $file`;
}