$content =~ s/(\w+)(: *)(.*)\n<xxxxyyxxxx>\n/$1.":".$2.encode_base64($3,'')."\n"/ges;

##</code><code>##

cn: dn=toto
sn: XXX
description: YYY

##</code><code>##

cn: dn=toto
sn: XXX
description: YYY
yyyy
<xxxxyyxxxx>

cn: dn=toto2
sn: XXX2
description: YYY2

cn: dn=toto3
sn: XXX3
description: YYY3
tttt
uuuuuu
<xxxxyyxxxx>

##</code><code>##

$content =~ s/(\w+)(: *)(.*)\n<xxxxyyxxxx>\n/$1.":".$2.encode_base64($3,'')."\n"/ges;

##</code><code>##

$content =~ s/(\w+)(: *)([^\n]*)\n<xxxxyyxxxx>\n/$1.":".$2.encode_base64($3,'')."\n"/ges;