Without using any of the 5.10 features you can do:

$string = 'bla bla [[en:English]][[de:German]][[ga:Irish]] bla bla';

%matches = $string =~ m/\[ \[ ( en|de|ga ) : (.+?) \] \]/gx;
$var = { lang => [ keys %matches ] };

pp $var;

{ lang => ["en", "ga", "de"] }
[download]

But that makes me wonder why you are capturing the longnames just to throw them away?

If you remove the capture for those you can do:

my $string = 'bla bla [[en:English]][[de:German]][[ga:Irish]] bla bla'
+;

my $var = { lang => [ $string =~ m/\[ \[ ( en|de|ga ) : .+? \] \]/gx ]
+ };

pp $var;

{ lang => ["en", "de", "ga"] }
[download]