#! perl -slw
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 more junk update junk and 5843 and 1522 mark '
    . 'complete update 8435 and 9323 mark complete true junk'
;

$data_stg =~ s[update ((?:\d+|\s|and)+) mark complete]{
    join ' ', map{ "update $_ mark complete"} split '\s+and\s+', $1
}ge;

print $data_stg;

__END__
## Output wrapped to match input for easier verification.
junk text update 8923 mark complete update 8324 mark 
complete more junk update 5438 mark complete update 5843 mark complete update 1522 mark 
complete more junk update junk and 5843 and 1522 mark 
complete update 8435 mark complete update 9323 mark complete true junk

##</code><code>##

#! perl -slw
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 more junk update junk and 5843 and 1522 mark '
    . 'complete update 8435 and 9323 mark complete true junk'
;

$data_stg =~ s[(update (.+?) mark complete)]{
    my @numbers = split '\s+and\s+', $2;
    if( grep{ !/^\d+$/ } @numbers ) {
        warn "Malformed request: '$1'\n";
        $1;
    }
    else{
        join ' ', map{ "update $_ mark complete"} @numbers;
    }
}ge;

print $data_stg;

__END__
## Output wrapped to match input for easier verification.
Malformed request: 'update junk and 5843 and 1522 mark complete'

junk text update 8923 mark complete update 8324 mark
complete more junk update 5438 mark complete update 5843 mark complete update 1522 mark
complete more junk update junk and 5843 and 1522 mark
complete update 8435 mark complete update 9323 mark complete true junk