Thought it might be helpful to see a rewrite of the code. ;)

Just cleaned up a bit, and added use strict; and use warnings; and a little bit of error handling.

#!/usr/bin/perl

use strict;
use warnings;

my $fsearch = 'file.txt';
open (TEXTFILE, 'c:/perl_programs/' . $fsearch) or die "CAN'T OPEN $fs
+earch:  $^E\n";
print "Enter a string to search for:  ";
chomp (my $search = <STDIN>);
my @line = <TEXTFILE>;
my $count = 0;
foreach (@line){ 
  $count++;
  if  ($_ =~ /$search/i)  {  
    print "\nLine that matched <$search> found on line #$count\n"; 
    print "$_\n";
  } 
}

if (!$count) {
  print "\nsubstring <$search> NOT found\n";
}
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