I'm not sure I fully understand your problem but try this

#!perl
use strict;
# test
for ('file1LAY.html','file1DFT.html','file2DFT.html'){
  open OUT,'>',$_;
  createButtons(undef,$_);
}

sub createButtons{
  my ($outfile,$filename) = @_;
  my %file=();
  
  if ($filename =~ m/(.+)(?:LAY|DFT)\.html/){
    for my $s ('LAY','DFT'){
      $file{$s} = './'.$1.$s.'.html';
      $file{$s} = '#' unless (-e $file{$s});
    }
  }
  
  print qq!
     <a href="$file{'LAY'}" class="button_lay">Layout</a>
     <a href="$file{'DFT'}" class="button_dft">DfT</a>\n
  </div>\n!;    
}
[download]

poj