Wanted write a warning thread ... but of course PM had it already, so I just need pushing it up again. =)

Shortly stated: When composing regexes from smaller parts be aware that " " and "#" are real new metacharacters under /x and not only syntactic sugar on the top level.

  DB<135> $a='#x'
 => "#x"

  DB<136> "x#x#x" =~ s/$a$a/-/r
 => "x-"

  DB<137> "x#x#x" =~ s/$a$a/-/xr  #oops
 => "-x#x#x"
[download]

"$a$a" becomes "#x#x" which is an empty regex under /x since it starts with a comment.

One solution¹ is to pre-compile the smaller parts w/o x-flag

  DB<138> $a=qr/#x/
 => qr/#x/

  DB<139> "x#x#x" =~ s/$a$a/-/r
 => "x-"

  DB<140> "x#x#x" =~ s/$a$a/-/xr
 => "x-"
[download]

another one escaping or using a character class

  DB<144> $a='\#x'           # or ='[#]x' 
 => "\\#x"

  DB<145> "x#x#x" =~ s/$a$a/-/r
 => "x-"

  DB<146> "x#x#x" =~ s/$a$a/-/xr
 => "x-"
[download]

simply using quotemeta might bite you again when you wanted to use other metacharacters.

NB: same problem with whitespace.

¹) IMHO the cleanest and still unmentioned in this thread :)