use strict;
my $file_name="c:\\temp.txt";
open (FH,"$file_name");
my $pattern="some thing";
my $c=0;
my $space=' ';
my $line;
while(<FH>)
{
	$line=$_;				#current line
	$c++;					#counts the number of lines
	$space=' ' x $c .' ' x length($pattern); #pattern that has to be replaced
	$line=~s/$pattern/$space/i;
}