sub foo {
    if ( 1 ) {
        ONCE();
        # This block is run only once, ever.
    }
}

use Devel::Caller 'caller_cv';
use B 'svrev_2object';
use B::Utils qw( walkoptree_filtered opgrep );
use B::Generate ();

sub ONCE {
    walkoptree_filtered
        svref_2object( caller_cv 1 )->ROOT,
        \ &find_ENTERSUB,
        \ &removE_ONCE;
}

sub remove_once {
    # Remove the parent of this op from execution.
    my $op = shift;
    my $parent = $op->parent;

    # This ignores other nodes which might point to this one using ->other().
    $parent->previous->next( $parent->next );

    # Remove $parent from the family tree.
    my $grandma = $parent->parent;
    if ( ${$grandma->first} == $$parent ) {
        $grandma->first( $parent->sibling );
    }
    else {
        for my $aunt ( $grandma->kids ) {
            if ( ${$aunt->sibling} == $$parent ) {
                $aunt->sibling( $parent->sibling );
                last;
            }
        }
    }
}

sub find_ENTERSUB {
    # I have no idea if this is the *right* ONCE() to remove.
    my $op = shift;

    # perl -MO=Concise -e "foo()"
    # (entersub
    #   (pushmark
    #     (ex-list
    #       (pushmark)
    #       (gv "foo"))))

    opgrep { name => 'entersub',
             first => { first => { sibling => { first => { PV => 'foo' } } } } },
           $op;
}