@EXPORT= qw(                       
    XML_FOO
    XML_BAR
);  

open my $foo, '<', \<<FOO;
#XML CODE HERE
FOO

use constant XML_FOO => \$foo;

open my $bar, '<', \<<BAR;
#XML CODE HERE
BAR

use constant XML_BAR => \$bar;

# and so on...

# Later in the implementation:

$parser->parse_string(${XML_FOO()});