It is as zer says. There are characters in your variable that are special to the regular expression engine (namely | and +). An alternative to manually back-whacking them would be to use quotemeta or the \Q "operator" within the regular expression. Here are two examples that work the same ...

Example 1:

$RTBL_ENTRY_DEL = quotemeta $RTBL_ENTRY_DEL;
# ... and then later ...
s/$RTBL_ENTRY_DEL//g;
[download]

Example 2:

s/\Q$RTBL_ENTRY_DEL//g;
[download]

I prefer the latter for what I believe are obvious reasons :-)