use strict;
use warnings;

while ( <DATA> ) {
    chomp;
    next unless $_;
    print "$_\t=>\n";
    s{(?<!\w:)//}[/]g;
    print "$_\n\n";
}

__DATA__
http://www.perlmonks.org/index.html
http://www.perlmonks.org//index.html
http:///davido.perlmonk.org/index.html
http://www.yahoo.com:8080/index.html