say "foo: bar" =~ s/^ ( \w+: \s* ) bar /${1}baz/xr;

##</code><code>##

say "foo: bar" =~ s/^ \w+: \s* \K bar /baz/r;

##</code><code>##

say "a:b:c:d" =~ s/ :\K [^:]+ (?=:) / uc($&) /xegr;

##</code><code>##

say "a:b:c:d" =~ s/ : [^:]+ (?=:) / ":" . uc($&) /xegr;

##</code><code>##

say "foo: bar" =~ s/ (?<= ^ \w+: \s* ) bar /baz/xr;

##</code><code>##

say "foo: bar" =~ s/^ \w+: \s* \K bar /baz/r;