main_file.pl
-------------------
use strict;
use Env;
use Getopt::Long;
use Config::Properties;
use Net::FTP::Recursive;
use Net::SFTP::Foreign;
use File::Copy;
use File::Basename;
use Cwd;
use Fcntl ':mode';
use MIME::Base64::Perl;
use Net::MySQL;

...
  if(something){
	require("sub_file.pl");
	sub_from_sub_file();
  }
...


sub_file.pl
-------------------
use strict;
use XML::LibXML;
use Config::Properties;
use DBI;
use threads;

...
sub_from_sub_file(){
   print "Before\n";
   my $thr = threads->new(\&sub1);
   print "After\n";
}

sub sub1{
	print "HI!\n";
}
...