quotemeta 
use warnings;
use strict;

my $old = quotemeta "welcome|to|chennai";
my $new = "Capital of karnataka|is|bangalore";

my $base = ();
my @base = ();
@base = <DATA>;
chomp @base;

foreach $base (@base) {
    if ( $base =~ /$old/ ) {
        $base =~ s/$old/$new/gi;
        print("Replaced!\n");
    }
    print @base;
}

__DATA__
welcome|to|chennai

[download]

The | is a special character in regexes (see perlre). It is used for alternation. The s///g modifier was replacing "welcome" with your $new string, then replacing "is" with your $new string, then replacing "chennai" with your $new string.

I've also chomped your input.

In reply to Re: search and replace if pattern found in file by toolic
in thread search and replace if pattern found in file by myfrndjk

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.