You can't. Just use the normal expression :

foreach (@input) {
       if( exists $list{ $_ } ) {
            push @name_nums, $list{$_};
       } else {
            push @name_names, $_;
       }
}
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TMTOWTDI :

foreach my $input (@input ) {
     push @name_nums, ( exists $list{ $input } ? $list{ $input } : $in
+put );
}
## following will not work if $list{ $input } can be 0 or undef,
## but here's another way
foreach my $input ( @input ) {
     push @name_nums, ( $list{ $input } || $input );
}
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