Here's an improved version of the code I posted in the CB in answer to your question.

my $replacestring = "TOM";
open(my $infile,"<","inputfile.txt") or die $!;
open (my $outfile,">","outputfile.txt") or die $!;
while (<$infile>)
{
    $_=~s/$replacestring//g;
    print $outfile $_;
}
close $infile;
close $outfile;
[download]

That will read the content of 'inputfile.txt', and write out the content to 'outputfile.txt', removing $replacestring along the way. The key bit is $_=~s/$replacestring//g - that replaces any instance of $replacestring in the $_ variable with blank.

The remaining step is to rename 'inputfile.txt' as something else, and rename 'outputfile.txt' as 'inputfile.txt'.

Update: Something else you might want to look at: your post says that you want to remove a string, but later you say a line. This code removes the entire line - I'll let you work out why ;-)

my $string = "TOM";
open(my $infile,"<","inputfile.txt") or die $!;
open (my $outfile,">","outputfile.txt") or die $!;
while (<$infile>)
{
    if ($_ !~/$string/)
    {
       print $outfile $_;
    }
}
close $infile;
close $outfile;
[download]