1 #!/usr/local/bin/perl
  2 use strict;
  3 use warnings;
  4 use LWP::Simple;
  5 use HTML::TreeBuilder::XPath;
  6 use Data::Dumper;
  7 use feature 'say';
  8 my $url="http://www.ebi.ac.uk/thornton-srv/databases/cgi-bin/pdbsu
+m/GetPage.pl?pdbcode=1r9t&template=main.html";
  9
 10 my $p = HTML::TreeBuilder::XPath->new_from_content(get($url));
 11
 12 my @chain_tags = $p->findnodes("//td//a[contains(\@href,'chain=')]
+");
 13 my $uniq_chains = { map { $_->attr('href') =~ /chain=(\w)/; ($1,1)
+; } @chain_tags };
 14 say " Number of chains : " . scalar keys %$uniq_chains;
 15 say keys %$uniq_chains;
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I used your code and add the function split

#!/usr/local/bin/perl
use strict;
use warnings;
use LWP::Simple;
use HTML::TreeBuilder::XPath;
use Data::Dumper;

my $url="http://www.ebi.ac.uk/thornton-srv/databases/cgi-bin/pdbsum/Ge
+tPage.pl?pdbcode=1r9t&template=main.html";
my $p = HTML::TreeBuilder::XPath->new_from_content(get($url));
my @chain_tags = $p->findnodes("//td//a[contains(\@href,'chain=')]");
my @chains = map { $_->attr('href') =~ /chain=(\w)/ } @chain_tags;
my @chain = map {split ' '} @chains;
foreach my $chain (@chain){print "$chain\n";}
[download]

the output was:

pisolo@pisolo:~/Scrivania/test$ perl test7.pl 
A
A
B
C
E
F
H
I
J
K
L
[download]

so I think I can iterate my analysis for every chain....using as array with foreach....Thanks.... :-)