You can easily make that safe by substituting a list of indexes for the actual items:

my @arr = qw/a b c/;
 foreach ( 0 .. $#arr ) {
     $_ = $arr[ $_ ];
     push @arr, 'd' if $_ eq 'a';
     push @arr, 'e' if $_ eq 'b';
     push @arr, 'f' if $_ eq 'c';
     print $_;
 }
 print "\n";
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