My idea for your problem looks like this:

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


my $datafile          = 'account.data';
my $searched_customer = 'customer2';

# modifications of values
my %modify = (
  credit =>  -100,
  debit  =>  250,
);

my %indent = (
  bakjob   => ' 'x2,
  type     => ' 'x4,
  customer => ' 'x6,
);

#
my %re = (
  bakjob   => qr{^$indent{bakjob}bakjob(\d+)_details},
  type     => qr{^    (debit|credit) =},
);


#open my $fh, '<', $datafile or die $!;
my $fh = *DATA;

# don't start before this line
my $line_num = 4;


while ( my $line = <$fh> ) {

    # only parse complete bakjob blocks after line $line_num
    if ( $. > $line_num
      && $line =~ m/$re{bakjob}/ .. $line =~ m/^$indent{bakjob}},?/ 
    ) {

        # parse internal structure
        if ( $line =~ m/$re{type}/ ) {
            print $line;
            parse( $fh, $1 );
        }
        else {
            print $line;
        }
    }

    # maybe this is wanted, if you need to get all lines printed:
    # if ( $. < $line_num ) {
    #    print $line;
    # elsif ( $line =~ m/$re{bakjob}/ .. $line =~ m/^$indent{bakjob}},
+?/ ) {
    #    # parse internal structure
    #    ....
    # }
    # else {
    #   print $line;
    # }
}

sub parse {
    my ( $fh, $type ) = @_;

    while ( my $line = <$fh> ) {
        chomp $line;
        my ( $customer, $value ) = split m{\s*=\s*|,}, $line;

        $customer =~ s/^\s+//;

        if ( $customer eq $searched_customer ) {
            printf "$indent{customer}%s = %.1f\n", $customer, $value+$
+modify{$type};
        }
        else {
            print $line, "\n";
        }

        last if $line =~ m/^$indent{type}},?/;
    }
}

__DATA__
  bakjob1_details = {
    credit = {
      customer1= 2000.0,
      customer2 = -1500.0
      customer3 = 0.0,
    },
    debit = {
      customer1= 50000.0,
      customer2 = -2000.0,
      customer3 = 0.0,
    }
  },
  bakjob2_details = {
    credit = {
      customer1= 1000.0,
      customer2 = 200.0,
      customer3 = 500.0,
    },
    debit = {
      customer1= 600.0,
      customer2 = 659.0,
      customer3 = 887.0,
    }
  }
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