So, applying the idea I linked to above, we get this.

$_ = "t?e\\\\xt\\\\* with escapes\\*\n"; 
s/\\\\/\\s/g, s/(?<!\\)\?/#/g, s/(?<!\\)\*/?/g, s/\\(\W)/$1/g, s/\\s/\
+\/g; 
print;
[download]

This outputs

t#e\xt\? with escapes*
[download]

Update: without lookarounds (this is what you'd do if all you had was sed):

$_ = "t?e\\\\xt\\\\* with escapes\\*\n"; 
s/\\\\/\\s/g, s/\\\?/\\S/g, s/\\\*/\\T/g, s/\?/#/g, s/\*/?/g, s/\\(\W)
+/$1/g, s/\\S/?/g, s/\\T/\*/g, s/\\s/\\/g; 
print;
[download]