Using a Here-doc like that is just like using a double quoted string, and fuction calls don't interpolate inside a double quoted string without some (IMHO ugly) "magic". You could try something like:

print SENDMAIL << "EOF";
From: me\@abc.com
To: you\@xyz.com
EOF
print SENDMAIL fuction(arguments);

close(SENDMAIL);
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Or assign the return value of the function to a variable, and put that inside the heredoc to interpolate normally.

my $text = function(arguments);

print SENDMAIL << "EOF";
From: me\@abc.com
To: you\@xyz.com
$text
EOF

close(SENDMAIL);
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