It's because that example is using die (vs. print) and the way die needs \n to avoid printing $! (which will affect the comparison against that die message later in that example).

The OPs problem is just buffering. Either 

$| = 1;
$SIG{ALRM} = sub {print "checking"};

[download]
or 
$SIG{ALRM} = sub {print "checking\n"};

[download]
fixes his problem.

In reply to Re^2: How to use alarm? by broomduster
in thread How to use alarm? by stephenp

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