I think grep might help you out, especially if you need to check against all of the @erasethese items. Since you did not provide a small sample of your input file, I will only offer this untested code (which does check against all @erasethese items):

sub GROOMLDIF {
    # Delete LDAP-internal fields
    my @erasethese = qw(
        structuralObjectClass
        entryUUID
        creatorsName
        modifiersName
        createTimestamp
        modifyTimestamp
        entryCSN
    );
   
    open my $fh_in , '<', "data/all.ldif"     or die "can not open dat
+a/all.ldif: $!";
    open my $fh_out, '>', "data/groomed.ldif" or die "can not open dat
+a/groomed.ldif: $!";
    while (<$fh_in>) {
        my $line = $_;
        my @thing = (split /:/)[0];
        print $fh_out $line unless (grep {$thing eq $_} @erasethese);
    }
}
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