You didn't describe the problem in sufficient detail.

Do you really get John, if it's there?

$str =~s/.*(John).*/$1/;
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Do you want to keep the last string after the last slash and before the dot?

$str =~s{.* / | \. .*}{}gx;
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Or, maybe, you want the fist word after the fifth slash?

$str =~s{ (?: [^/]* / ){5} ( \w+ ) .* }{$1}x;
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Or, ...

Also, do you really need to use substitution? Matching seems more appropriate.

print $str =~ /(John)/;
print $str =~ m{ .* / (.+) \. }x;
print $str =~ m{ (?: [^/]* / ){5} ( \w+ ) }x;
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Do you need to use regexes at all?

my $s = 'John';
print $s if -1 != index $str, $s;
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Update: Some alternatives provided.