In your example you don't have a clear order in which the pattern can happen so the second part of the code will be useful. If the order is set then you can use the first part of the code

#!/usr/bin/perl -w

$string = "Test: FileNames can't have / \\ and other special character
+s";

$string =~ s/(.*):(.*)\/(.*)\\(.*)/$1COLON$2FSLASH$3BSLASH$4/; # Note 
+greedy substituion in place

print $string,$/;

my %subs = qw(: COLON / FSLASH \\\\ BSLASH);

$string =~ s/$_/$subs{$_}/ for (keys %subs);

print $string,$/;
[download]

Output

TestCOLON FileNames can't have FSLASH BSLASH and other special charact
+ers
TestCOLON FileNames can't have FSLASH BSLASH and other special charact
+ers
[download]