Break it down:

1. grep( expression, @{ ... } )
2. The expression  s#$#$RegObj->{DELIM}#

   start with 's', so it is a substitution.

   The #s are delimiters so the two sides are: $ and $RegObj->{DELIM}

   So it is substituting the module defined delimiter for any '$'s every end of line.

3. The array: @{ $RegObj->{MEMBERS}= [ @{$RegObj->{SUBKEYS}} ] }

   Contains an assignment: $RegObj->{MEMBERS}= [ @{$RegObj->{SUBKEYS}} ]

   It copies the array of subkeys : @{ $RegObj->{SUBKEYS} } into an anonymous array [ ... ]

   And assigns its reference to $RegObj->{MEMBERS} = ...

   Then it dereferences that to supply the subkeys to grep.

Thus, that one line is equivalent to:

my @temp = @{ $RegObj->{SUBKEYS} };
$RegObj->{MEMBERS} =  \@temp;
grep s[$][$RegObj->{DELIM}], @temp;
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Without the need for @temp; but less efficient.