open INFILE,  "<" . $filename 
     or die "Couldn't open file " . $filename . " for reading : $!";
open OUTFILE, ">" . $filename 
     or die "Couldn't open file " . $filename . " for writing : $!";
while (<INFILE>){
	print OUTFILE $_;
	last if /$heading_to_print/;
}
while (<INFILE>){
	print OUTFILE $_ unless /$heading_to_print/;
}
close INFILE;
close OUTFILE;

##</code><code>##

open INFILE,  "<" . $filename 
     or die "Couldn't open file " . $filename . " for reading : $!";
open OUTFILE, ">" . $filename 
     or die "Couldn't open file " . $filename . " for writing : $!";
my %hash;
while (<INFILE>){
	print OUTFILE $_ unless $hash{$_}++;
}
close INFILE;
close OUTFILE;

##</code><code>##

perl -ni.old -e "print $_ unless $_{$_}++" foo.txt