print header('application/pdf'), do {
    open my $fh, '<:raw', 'minimal.pdf' or die "D'Oh! $!\n";
    local $\;
    <$fh>;
};
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Due to the funny way CGI::header() operates under mod_perl it is advisable to always print the result of CGI::header immediately and never defer it. In this particular case it probably doesn't matter but one day it may.

print header('application/pdf');
{
    open my $fh, '<:raw', 'minimal.pdf' or die "D'Oh! $!\n";
    local $\;
    print <$fh>;
};
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Note: the above code slurps the whole PDF into memory. I prefer to use File::Copy

use File::Copy 'copy';
print header('application/pdf');
copy 'minimal.pdf', \*STDOUT or die "D'Oh! $!\n";
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