has file => ( isa => 'ExistingFile', is => 'ro', required => 1, coerce => 1 );

##</code><code>##

You cannot coerce an attribute (file) unless its type (ExistingFile) has a coercion at /tmp/test.pl line 26

##</code><code>##

subtype 'ExistingFile' =>
    as 'AFile' =>
    where { say "In subtype where clause: $_[0]"; -f $_[0] },
    message { "$_[0] does not exist" };

coerce  'ExistingFile' =>
    from 'Str' =>
    via { file($_) };

has file => ( isa => 'ExistingFile', is => 'ro', required => 1, coerce => 1 );

##</code><code>##

for my $ftype (qw/ AFile ExistingFile /) {
    coerce  $ftype =>
        from 'Str' =>
        via { file($_) };
}