So here is the code

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 r
+eplaced
    $line=~s/$pattern/$space/i;
}
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