c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'foo\bar\[baz[x]x{x}x~x*x?x[]{}~*?';
 print qq{'$s'};
 ;;
 $s =~ s{ (?= [][{}~*?]) }{\\}xmsg;
 print qq{'$s'};
"
'foo\bar\[baz[x]x{x}x~x*x?x[]{}~*?'
'foo\bar\\[baz\[x\]x\{x\}x\~x\*x\?x\[\]\{\}\~\*\?'