Replace

$msg .= "cannot call $q\n";
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with

$msg .= "cannot call @{[eval {&$q}, $@ =~ /^[^&]+(\S+)/]}\n";
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and the output becomes

hello from ABC(hhhhhh)
cannot call &main::def
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You can use

@{[eval {&$q}, $@ =~ /^[^&]+(\S+)/]}
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in any double-quoted string for whatever reporting purposes you want.

Thanks to BrowserUk for the eval bit (above).