Yes, perl interpolates double quoted strings, including regular expression patterns used in the match and substitution operators.    The gory details are in: Gory details of parsing quoted constructs.

If you don't want string interpolation you can use single quoted operators:

if ( $hello =~ m'^[ \|\t\r\n]+$' ) {
[download]

But then "\t", "\r" and "\n" won't be interpolated.