According to perldoc -f system,

 Because "system" and backticks block "SIGINT" and
 "SIGQUIT", killing the program they're running
 doesn't actually interrupt your program.
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However, I find that I get different results using backticks compared to system:

use strict;
use warnings;

my $Abort=0;
$SIG{INT} = sub { ++$Abort };

for (1..10) {
  print "Iteration $_: Abort = $Abort\n";
  `sleep 5`;
}
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$Abort will increment if I use backticks, not if I use system.