You was very close there: 
perl -MTk -we "$m=tkinit;$c=$m->Canvas->pack(-fill=>'both',-expand=>1)
+;$m->Button(-command=>sub{print $c->width})->pack;MainLoop;"

[download]
But its not easy to recalculate all item's X and Y positions.

In reply to Re: Automatic canvas content rescaling by vkon
in thread Automatic canvas content rescaling by TheMarty

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