You can always disable the warning if it bothers you. Of course, its only a significant gain if the alpha appears early in the file. And, the sub overhead may kill the gain, if the processing involved isn't very significant:

#! perl -slw
use strict;

sub doBeta(_) {
    print 'Beta';
}

sub doit(_) {
    if( /alpha/ ) {
        print 'Alpha';
        *doit = \&doBeta;
        return;
    }
    doBeta( $_ );
}

doit( $_ ) while <DATA>;

__DATA__
beta
beta
beta
beta
beta
beta
beta
beta
beta
alpha
beta
beta
beta
beta
beta
beta
beta
beta
beta
beta
beta
beta
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