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: $!";