my $failed;
eval {
    local $SIG{ALRM} = sub { die "TIMEOUT\n" }; # N.B. \n required
    eval {
	alarm($seconds);
	... code to execute with timeout here (which may die) ...
	    1;
    } or $failed = 1;
    # outer eval catches alarm that might fire JUST before this alarm(0)
    alarm(0);  # cancel alarm (if code ran fast)
    die "$@" if $failed;
    1;
} or $failed = 1;
if ( $failed ) {
    if ( defined $@ and $@ eq "TIMEOUT\n" ) { ... }
    else { ... } # some other error
}