On my C2D 2GHz your code is executed in 4.4s with 5.10.1 and in 4.0s with 5.8.9. The following code takes about 3.7s:

@f = <>;
for (1..shift(@f)) {
    @s=();
    for (1 .. shift(@f)) {
        @l = split ' ', shift @f;
        $i=0;
        $y=0;
        for (@l) {
            $x = $y;
            $y = $s[$i++];
            $_ += $x > $y ? $x : $y;
        }
        @s = @l;
    }
    $m = 0;
    $m = $m > $_ ? $m : $_ for @s;
    print "$m\n";
}
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And this seems slightly faster:

for ( 1 .. <> ) {
    @s = ();
    for ( 1 .. <> ) {
        @l = split ' ', <>;
        ( $i, $y ) = ( 0, 0 );
        for (@l) {
            $x = $y;
            $y = $s[$i];
            $s[ $i++ ] = $x > $y ? $_ + $x : $_ + $y;
        }
    }
    $m = 0;
    $m = $m > $_ ? $m : $_ for @s;
    print "$m\n";
}
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