Try using the range operator - something like i.e. untested...

use warnings;
use strict;
use autodie;

open INF, "<$file";
while (<INF>){
    if(/First line of text$/ ... /in the third line I have (\d*\.\d*)$
+/)
    {
        push @array, $1 if $1;
    }
}
[download]

Note the use of both strictures and autodie - the latter causing the snippet to die if open() fails.