use Digest::SHA qw(sha1_hex);

local $/ = undef;
my $wiki_code = <DATA>;

$wiki_code =~ s/(\[\[Image:)(.+)\/(.+)(\]\])/"$1".sha1_hex($2)."_$3$4"/ge;
   
print $wiki_code;

__DATA__
[[Image:Vermessungspunkte-Dateien/image021.jpg]]
[[Image:Vermessungspunkte-Dateien/image028.jpg]]


Marker: [[Image:Vermessungspunkte-Marker/image011.jpg]]