Moose won't do coersion unless you request it, so, at the least, you would need:

has file => ( isa => 'ExistingFile', is => 'ro', required => 1, coerce
+ => 1 );
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However, that doesn't work for me either, complaining:

You cannot coerce an attribute (file) unless its type (ExistingFile) h
+as a coercion at /tmp/test.pl line 26
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Apparently, coersion is not inherited by subtypes by design, so you will need to duplicate the coersion.

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 );
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Note: You can reduce the code duplicationg using:

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