Here's some code to get you going:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = qq{[5:0]abc\n[0:5]def\nghi\njkl\n[22:22]mno\n[6:3]pqr\nstu\n}
+;
 print qq{<<$s>> \n};
 ;;
 $s =~ s{ (\[ \d+ : \d+ \]) ([[:lower:]]{3}) }
        { make_replacement($1, $2)           }xmsge;
 print qq{<<$s>>};
 ;;
 sub make_replacement {
   my ($range_field, $string) = @_;
   ;;
   my ($hi, $lo) = $range_field =~ m{ \d+ }xmsg;
   return qq{$range_field$string} if $hi < $lo;
   ;;
   return
     join qq{\n},
     map  qq{$string\[$_]},
     $lo .. $hi
     ;
   }
"
<<[5:0]abc
[0:5]def
ghi
jkl
[22:22]mno
[6:3]pqr
stu
>>

<<abc[0]
abc[1]
abc[2]
abc[3]
abc[4]
abc[5]
[0:5]def
ghi
jkl
mno[22]
pqr[3]
pqr[4]
pqr[5]
pqr[6]
stu
>>
[download]

Still some questions to answer:

• Is the 'abc' string portion of '[5:0]abc' always lower case? Always exactly three digits?
• What should happen if the "range" is inverted as in '[0:5]def'? What if it is null: '[22:22]mno'?

Update: Added the  '[6:3]pqr' instance.