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

use IO::All;
use Data::Dumper;
use feature 'say';

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

for my $i (0 .. $#lines) {
    if ($lines[$i] =~ /operating_conditions/) {
	# Insert at position 12, replace 0 elements.
	splice @lines, $i + 5, 0, 'default_operating_conditions : "AB0.5v45c" ;';
	last; # break loop
    }
    next;
}

print Dumper \@lines;

__END__

$ perl test.pl
$VAR1 = [
          '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 ;',
          '  }',
          '',
          '  input_voltage(default) {',
          '    vi : 0 ;',
          '    vh : 0.5 ;',
          '    vim : 0 ;',
          '    vin : 0.5 ;',
          '  }',
          '',
          '}'
        ];
$VAR1 = [
          '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 ;',
          '  }',
          '',
          '}'
        ];