#!/usr/bin/perl
use strict;
use warnings;

use IO::All;
use Data::Dumper;

my @lines = io('file.txt')->chomp->slurp;

for my $i (0 .. $#lines) {
    if ($lines[$i] =~ /operating_conditions/) {
	# split line based on white space
	my @elements = split /\s+/, $lines[$i];
	# remove first and last character of the string
	$elements[2] =~ s/^.(.*).$/$1/;
	# Insert string at position 12, replace 0 elements.
	splice
	    @lines,
	    $i + 5,
	    0,
	    "  default_".$elements[1]." : ".$elements[2]."";
	last; # break loop
    }
    next;
}

# print Dumper \@lines;
io('out.txt')->appendln($_) for @lines;

__END__

$ perl test.pl
$ cat out.txt
library(and_gate) {
  delay_model : table_lookup ;
  date : "Fri Mar 15 03:44:39 " ;
  time_unit : 1ms ;
  voltage_unit : 1V ;
  current_unit : 1A ;

  operating_conditions       ("AB0.5v45c") {
    process : 1 ;
    temperature : 45 ;
    voltage : 0.5 ;
  }
  default_operating_conditions : "AB0.5v45c"

  input_voltage(default) {
    vi : 0 ;
    vh : 0.5 ;
    vim : 0 ;
    vin : 0.5 ;
  }

}