#!/usr/bin/perl

use strict;
use warnings;

sub mysub{
#Do some clean up before exit.
};

my $fh;
unless (open ( $fh, '>' , "file.txt")) {
mysub();
print "Cannot open file.txt for writing.\n"; 
exit 0 ; 
};


my $condition = 0;
do { 
	my $text = "Text to be written over file.txt.\n";
	print { $fh } "$text";
	#... other stuff goes on, eventually changing $condition to 1
} until ($condition);

close ($fh);
mysub();

exit 1;