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use constant DEFAULT_FILE => "/etc/filesystems";
my $file = shift || DEFAULT_FILE; # e.g. to take file from command lin
+e or use the default
...
  my ($file, $pattern) = @_;
  # your code here
}

...
die "oh no, it's not there!" unless find_pattern($file, $pattern);
...
  my ($file, $pattern) = @_;
  return `grep $pattern $file`;
}