perl -wle '$foo = "a hello w"; $foo = $foo =~ s/(?=.*(hello)).*/$1/sr; print $foo'

##</code><code>##

hello

##</code><code>##

...; print $foo =~ s/(?=.*(hello)).*/$1/sr;

##</code><code>##

...; $foo =~ /hello(?{ $foo = $& })/; print $foo