I want to assign the resulting string to a new var, I want to leave $sentence unchanged.
If you don't want to modify the original var, then don't use a substitution regex.
my $sentence = 'The quick brown fox jumps.';
my ($sentence2) = $sentence =~ /\s(quick)\s/;
print $sentence2, "\n";
I may have misinterpreted your goal, so if that doesn't do what you want, then please clarify your need.