Tested code, employing the Schwartzian_transform

use warnings;
use strict;
use Data::Dumper;

my @lines;
chomp(@lines = <DATA>);
my @sortedlines =   map  { $_->[0] }
                    sort { $a->[1] <=> $b->[1] }
                    map  { [ $_, (split /;/)[0] ] } @lines;
print Dumper(\@sortedlines);

__DATA__
.0000000001;2;1;.5;.2
.5;2;1;.5;.5
0.3;1.5;.5;.5;.2
1;1;.7;1;1
.4;1;.4;1;.5
[download]

Outputs:

$VAR1 = [
          '.0000000001;2;1;.5;.2',
          '0.3;1.5;.5;.5;.2',
          '.4;1;.4;1;.5',
          '.5;2;1;.5;.5',
          '1;1;.7;1;1'
        ];
[download]

See also:

• perldoc -q sort
• Where should I post X?: You originally posted this in PMD and I moved it to SoPW