C:\>perl
$pattern = "\\";
$variable = "abcde\\fg";
print $variable =~ /$pattern/;
^D
Trailing \ in regex m/\/ at - line 4.

##</code><code>##

$variable =~ s/
  (  # start capturing to $1
    (?: # not capturing, just clustering
      \Q$pattern\E # $pattern in a quoted way
    )
    + # one or more times
  )  # end capturing to $1
/
....
/x;

##</code><code>##

  $variable =~ s/((?:\Q$pattern\E)+)/$1.$1.$1/e;
  print $variable;