This is the proper syntax:

sub cleaner {
   my ( $text ) = @_;
   $text =~ s/\n/<br>/gs;
   $text =~ s/\t/ /g;
   return $text;
}

my $details = cleaner( $clean_me );
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Note that function prototypes don't work in perl like in C or PHP code. You can't say sub cleaner( $dirty ) and expect the function to contain a variable called $dirty with your input parameter - you have to pull it off the magic @_ parameter array using shift or the syntax I use here.

Perl does have function prototypes of a sort, but they serve a different purpose than in other languages

Also, if you are matching newlines in your regular expression, you will need the s flag - otherwise the match will stop at the first newline. The s flag tells Perl to treat newlines like any other character, and match to the end of the string.

Update: See below for why that last piece of advice is irrelevant - thanks, jdporter!.