This is "easy enough" if you consider the problem as the function ONCE() alters the optree of the function that calls it so that the scope that contained it is edited right out. Why not, I suppose.

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 ->o
+ther().
    $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;
}
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