This appears to do what you require:

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

if ( @ARGV != 1 ) {
    print "USAGE :: perl default_operating_condition_update.pl <<FILE>
+>\n\n";
    exit 1;
    }

my $inputfile = $ARGV[ 0 ];    # input  FILE

mkdir 'finallib' unless -d 'finallib';

open my $INFILE, '<', $inputfile or die "Can not open '$inputfile' bec
+ause: $!";
open my $OPFILE, '>', "finallib/$inputfile" or die "Can not open 'fina
+llib/$inputfile' because: $!";

while ( my $iline = <$INFILE> ) {

    print $OPFILE $iline;

    if ( $iline =~ /^ ( \s* ) operating_conditions \s* \( \s* ( .* ) \
+s* \) \s* { \s* $/x ) {

        my ( $leading_space, $operating_cond ) = ( $1, $2 );

        until ( $iline =~ /^ \s* } \s* $/x ) {
            $iline = <$INFILE>;
            print $OPFILE $iline;
            }

        print $OPFILE "${leading_space}default_operating_conditions : 
+$operating_cond ;\n";
        }
    }
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