There's also the more familiar options of either building the regex as a string first, or using temporary variables:

# Temp str
my $reg_str = '(\s'.getone().'\s'gettwo().')';
$str =~ /$reg_str/;

# Temp scalars
my $getone = getone();
my $gettwo = gettwo();
$str =~ /(\s $getone \s $gettwo )/xg;
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It's easier to understand than @{[ LIST ]} or (??{ CODE }) patterns, and probably not noticeably slower, if at all.