With non-destructive /r modifier (and lookahead):

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

hello
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And if you want only to use your capture, just:

...; print $foo =~ s/(?=.*(hello)).*/$1/sr;
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In the later example a value of $foo didn't change.
*Updated

+one more way:

...; $foo =~ /hello(?{ $foo = $& })/; print $foo
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Though it looks like bad practice to change a variable while it is in use.