You really should use an HTML parser to handle this job, but the following regular expression and substitution "trick" might just be robust enough to handle your data:

use strict;
use warnings;

my $data = do { local $/; <DATA> };

$data =~ s/
      (<input.*?ng-model=")
      ([^[]+)
      (\[\d+\]")
      (.*?)
  /
      $1
    . $2
    . $3
    . qq( label="@{[ (split '\.', $2)[-1] ]}")
    . $4
  /exgs
;

print $data;

__DATA__
<input type="checkbox"
    ng-model="data.mydebts.loan[0]"

<input type="checkbox"
    ng-model="foo.bar.baz.qux[2]" />

<input type="checkbox"
    ng-model="foo[300]" >
[download]

Results:

<input type="checkbox"
    ng-model="data.mydebts.loan[0]" label="loan"

<input type="checkbox"
    ng-model="foo.bar.baz.qux[2]" label="qux" />

<input type="checkbox"
    ng-model="foo[300]" label="foo" >

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)