To expand a bit on japhy's solution of using a hash:

  # set this up once...
  my %cols = ( UN  => 0, RN  => 1, LOC => 2, ... );
  my $cols = do { my $tmp = join '|', keys %cols; qr/$tmp/ };

  # for each prompt to fill in:
  $prompt =~ s/-($cols)-/$user_info[ $cols{$1} ]/g;
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This avoids the need for /e on the substitution as well as being very specific about what can be matched for template substitution.

Depending on what else you are doing with the @user_info array, you can easily convert it into a hash by using slice assignment:

  # once
  my @col_info = ( UN  => 0, RN  => 1, LOC => 2, ... );
  my %cols = @col_info;
  my @cols = do { my $i = 0; grep { $i = 1-$i } @col_info };
  my $cols = do { my $tmp = join '|', @cols; qr/$tmp/ };

  # for each @user_info
  my %user_info; @user_info{@cols} = @user_info;
  # now you can look up your values by name instead of position:
  print "username: $user_info{UN}\n";

  # and our substitution turns into this:
  $prompt =~ s/-($cols)-/$user_info{$1}/g;
[download]

A suggestion: use full-length words instead of abbreviations like "UN" and "RN". It doesn't cost that much, and it saves your users (and yourself, six months from now!) having to look them up.