I'd first think to try the following, but that may be perl6 interpolation talking:

my $index = \&login;

for (keys %{$_[0]}) {
  print qq[<TD WIDTH="$table_vars{$_}">\n$index->()\n</TD>\n];
}
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Generally, and definitely perl5, "@{[foo()]}" calls &foo as list context and interpolates the return value as with an array and "${\foo()]}" similarly, with scalar context and as a scalar.

For your particular example, however, I'd just use printf.

for (keys %{$_[0]}) {
  printf qq[<TD WIDTH="$table_vars{$_}"\n%s</TD>\n],
    $index->();
}
[download]

There is no need to escape quoting delimeters in Perl.