Using the OP data provided, the following work for me: 

use warnings;
use strict;

while (<DATA>) {
    chomp;
    for my $to_add (qw[_float _auxpc2]) {
        ( my $string = $_ ) =~ s/^(.+?)(\s+?.+?)$/$1$to_add$2/;
        print $string, $/;
    }
}

__DATA__
lvtnfet      cmos20lpm      lvtnfet_b      symbol d g s b PROPMAP m=m

[download]
Output 
lvtnfet_float      cmos20lpm      lvtnfet_b      symbol d g s b PROPMA
+P m=m 
lvtnfet_auxpc2      cmos20lpm      lvtnfet_b      symbol d g s b PROPM
+AP m=m

[download]
UPDATE:
"..I need to search for specific string and add two lines.."
then you can do like so: 
while(<DATA>){
    ...
    if (/^lvtnfet/) {
    ...
    else { print $_, $/; }
}

[download]

If you tell me, I'll forget.
If you show me, I'll remember.
if you involve me, I'll understand.
--- Author unknown to me

In reply to Re: Search and add lines into a file. Getting bareword error. by 2teez
in thread Search and add lines into a file. Getting bareword error. by freekngeek

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