For me both cases are consistent and throw warnings instead of dieing

 perl -version

This is perl, v5.10.0 built for i486-linux-gnu-thread-multi
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code:

BEGIN {
    $SIG{__WARN__}=sub { print "WARNING: $_[0] \n" };
}

$|=1;

sub fn1($);
fn1("output1");
sub fn1 (\@) {
    print while $_ = shift;
}


sub fn2($);
fn2("output2");
sub fn2 (\@) {
    print $_,$/ for @_;
}
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output:

$ perl /tmp/proto.pl 
WARNING: Prototype mismatch: sub main::fn1 ($) vs (\@) at /tmp/proto.p
+l line 11.
 
WARNING: Prototype mismatch: sub main::fn2 ($) vs (\@) at /tmp/proto.p
+l line 18.
 
output1output2
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If you want it do die try to catch the /^Prototype mismatch:/ in $SIG{__WARN__}.¹

UPDATE