sub drawPolygon {
    glBegin(GL_POLYGON);
##    glColor3f( @{ shift() }[ 0, 1, 2 ] ); ## Works
    glColor3f( @{ shift() } ); ## Fails
    glVertex3f( @$_ ) for @_;
    glEnd();
}
[download]

In the call that works glColor3f() receives 3 arguments. In the call that fails glColor3f() receives 4 arguments - which rightly warrants a "Usage" error. The following rendition of sub drawPolygon works fine for me:

sub drawPolygon {
    glBegin(GL_POLYGON);
    my $arg = shift;
    pop(@$arg);
##    glColor3f( @{ shift() }[ 0, 1, 2 ] ); ## Works
    glColor3f( @{ $arg } ); 
    glVertex3f( @$_ ) for @_;
    glEnd();
}
[download]

Update: Duh, you'd already worked it out for yourself. I missed the "Solved" and didn't pay attention to the comments.

Cheers,
Rob