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$ perltidy file
# simple example perltidy script
my $input = <STDIN>;
if ( open( FILE, "<$input" ) ) {
    while ( $file = <FILE> ) {

        # pointless loop!
        for ( my $i = 0 ; $i < 100 ; $i++ ) { print "."; }
        print "\n";
        push( @entries, $file );    # copy contents of file to memory
        $count++;                   # keep a counter
    }
    close(FILE);
}
else {
    die "Could not open file $file: $!\n";
}

Useful hints

Checking IP names and IPv4 addresses

Assume we have the name or IPv4 address in $hostname then one can use (nb the address does not exclude octets of >255).

unless(($hostname=~/(([a-z0-9]+|([a-z0-9]+[-]+[a-z0-9]+))[.])+/)#Name
    || ($hostname=~/ \b(?:\d{1,3}\.){3}\d{1,3}\b/)){            #IPv4 addr
  print "hostname=$hostname, not a valid IP name or address\n";
  exit 101;
}

If one wishes to fully check that the octets are correct (<255) and also get the value of each octet (into $s1..4) then one can use:

\b(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\b

If one does not need the values of each octet then a simpler expression will surfice:

NOT-SET

An alternative is to use a library module such as NetAddr::IP.

For IPv6 addresses (which are much more complex) use a module such as: Net::IPv6Addr, Regexp::IPv6,or NetAddr::IP (do a Google search with the name).

Rough template

There is a rough template of a perl script that creates/sets several useful variables (user, host, debug level) uses strict and -w, has USAGE information, ensures created files are accessible to others, processes options, has the disclaimer notice, etc. It is not meant to do anything useful but may be useful as a start.