What is the result of adding an integer to a filehandle?

The addition operator will ask for the numeric representation of its arguments, so they will be coerced into numbers if they're not already. So the real question is: "What is the result of numifying a filehandle?"

Many things are considered file handles in Perl

• IO
• References to IO
• Glob
• References to glob
• Variable names (sometimes)

In this case, you have a reference to a glob

$ perl -MScalar::Util=reftype -le'open my $fh, "<", \$buf; print refty
+pe($fh)'
GLOB
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When you numify a reference, you get the memory address at which the referenced object is located.

$ perl -le'open my $fh, "<", \$buf; print $fh; printf "0x%x\n", 0+$fh'
GLOB(0x814ec28)
0x814ec28
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So you are adding the address of the glob associated with the file handle.