Hi, you need to make the optional part optional:

perl -Mstrict -wlE '
my @str=("w7R9Kstc017510: split:","w7R9Kstc017510[1]: split:","w7R9Kst
+c017510[2]: split:");
for (@str) {
say "$_ matches" if /(\w+)(?:\[\d+\])?: split:/ 
#                               here ^
}
'
w7R9Kstc017510: split: matches
w7R9Kstc017510[1]: split: matches
w7R9Kstc017510[2]: split: matches
[download]

Hope this helps!