#!/usr/bin/perl -w
use strict;

my $data_stg
    = 'junk text update 8923 mark complete update 8324 mark '
    . 'complete more junk update 5438 and 5843 and 1522 mark '
    . 'complete update 8435 and 9323 mark complete true junk';


pos( $data_stg ) = 0;

my %mult_updates;
my $pass = 1;
while ( $data_stg =~ /(update \d+( and \d+)+ mark complete)/ig ) {
    my $mult_update_pos = pos( $data_stg );
print "$pass pos: '$mult_update_pos'\n";

    my $mult_update = $1;
print "$pass orig_mult_update: '$mult_update'\n";

    my $mult_update_length = length $mult_update;
print "$pass length: '$mult_update_length'\n";

    $mult_update =~ s/and (\d+)/mark complete update $1/gi;
print "$pass new_mult_update: '$mult_update'\n";

    $mult_updates{ $mult_update_pos - $mult_update_length }
        = [ ($mult_update_length, $mult_update) ];
}
continue {
    $pass++;
}

# Work backwards from the end of the string, doing substr
# on positions which have been identified as having code to
# replace.  Let the key define a starting position and the
# key's value contain an array ref describing the length
# of the target and the desired replacement text.
foreach ( sort {$b <=> $a} keys %mult_updates ) {
    substr(
        $data_stg,
        $_,
        $mult_updates{$_}->[0],
        $mult_updates{$_}->[1]
    );
}

print "\n$data_stg\n";