It defines a constant with the string in the HERE-doc as value.
to make it clearer one could write that in two steps: 
my $c = <<USAGE;
Usage:ftp.pl  [options] host:/path/to/directory 
Options:
         --user <user> Login name 
         --pass <pass> password 
USAGE 

#$c now holds the text between <USAGE and USAGE.
print $c;

use constant USAGEMSG => $c;

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 Update:
as igekami pointed out, the above wonīt work. That happens when you donīt test your posts :(.


holli, /regexed monk/

In reply to Re: Meaning of 'use constant USAGEMSG = > ...' by holli
in thread Meaning of 'use constant USAGEMSG = > ...' by rjsaulakh

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