Not sure what exactly you're trying to do, but it looks like you want pattern replaced with line number (starting at 1) + pattern length number of spaces. You also want to do this only once per line. In addition, the file is specified as being up to 100000 bytes, which means the file can be loaded into memory all at once instead of being read line by line. The following should work: 
use strict;
use warnings;

my $path = "c:\\temp.txt";
my $pattern = "some thing";
my $handle;

open($handle, $path);
my @data = <$handle>;
close($handle);

my $c = length($pattern);
for (@data) {
    $c++; $_ =~ s/$pattern/' ' x $c/ei;
}

open($handle, ">$path");
print $handle @data;
close($handle);

[download]

In reply to Re: Replacing the pattern to search for by TedPride
in thread Replacing the pattern to search for by agynr

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.