I have managed to diagnose there is LF in the middle of line and the normal CRLF in the end. how can i remove this LF.

You can use a negative look-behind assertion to remove each LF that is not immediately preceded by a CR:

#! perl
use strict;
use warnings;

my $string = qq{"Tap Rackmount for 3 units\x0A","USR4500-RMK         "
+\x0D\x0A};

print "Before:\n>>>$string<<<\n";

#    Dec Hex  Oct
# LF  10  0A  012
# CR  13  0D  015

$string =~ s/ (?<!\x0D) \x0A //gx;

print "After:\n>>>$string<<<\n";
[download]

Output:

12:49 >perl 679_SoPW.pl
Before:
>>>"Tap Rackmount for 3 units
","USR4500-RMK         "
<<<
After:
>>>"Tap Rackmount for 3 units","USR4500-RMK         "
<<<

12:49 >
[download]

See the section “Look-Around Assertions” in perlre#Extended-Patterns.

Hope that helps,