Hi, try this,

use strict;

my $arg = $ARGV[0];
open(IN,"$arg")||die("Can't open the input file for reading");
undef $/;
my $str = <IN>;
close(IN);

$str=~s#\$[^\$]+\$#&LimitReplace($&)#gei;

open(OUT,">$arg")||die("Can't open the output file for writing");
print OUT $str;
close(OUT);

sub LimitReplace{
 my ($line)=@_;
 $line=~s#\\limits##gi;
 return($line);
}
[download]

Vivid

please don't delete the original text of your posts after your questions have been answered. Fellow monks who happen to read the answers won't see what the original question was, and thus can't learn from the question / answer play any more. Just leaveyour orignal post (OP) as it is, and insert lines below it marked with "update:"

Please read the entry How do I change/delete my post? in PerlMonks FAQ.

Thank you.

OP text:

I want to replace "\limits" inside "$ a+b=\sum\limit_a^{n-j}$"

#########
open IN, "abc.xml", or die "$!\n";
open OUT, ">iabc.xml", or die "$!\n";

@txt=<IN>;

foreach $xabc (@txt) {
$find=~ m/\$([^\$]*)\$/g;
$find = "\\limits";
$replace = "";
$find = quotemeta $find;
$xabc =~ s/$find/$replace/g;
print OUT $xabc;
}
close($xabc);
[download]

Please Help.

Thanks
POP

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

I might slightly rewrite as follows:

open my $in, '<', 'abc.xml'
    or die "Can't read abc.xml: $!";
open my $out, '>', 'iabc.xml'
    or die "Can't write iabc.xml: $!";
while ( my $line = <$in> ) {
    $line =~ s/\\limits//g;
    print $out $line;
}
close $in  or die "close input: $!";
close $out or die "close output: $!";
[download]

I note that the string you say you're looking for ("\limits") is not in the example string you give ("$ a+b=\sum\limit_a^{n-j}$"). I also note that the example string appears to be TeX, so a non-Perl solution might be to redefine the macro.