create an ascii progress bar

J, 78×0.75 = 58.5

'w p'=:_".1!:1]3
1!:2&4('%',~":100*p)(i.@[-<.@-:@-)&#}'[]'0 _1}' |'{~(w*p)>i.w

$ echo -n 79 0.15 | jconsole test.ijs
[|||||||||||                          15%                                     ]
$ echo -n 25 0.76 | jconsole test.ijs
[||||||||||76%|||||     ]   
$ echo -n 39 -0.12
[                 _12%                ]
$ echo -n 25 7.6 | jconsole test.ijs
[|||||||||760%||||||||||]   

(Negative numbers in J are prefixed by _, not -. Luckily, dyadic ". can parse both formats.)

Perl, 96×¾ = 72

#!/usr/bin/perl -ap
formline'[@'.'|'x($F[0]-3).']',100*$F[1].'%';
$_=$^A;s# |(.)#$1//($-[0]<$F[0]*$F[1]?'|':$&)#eg

That's by traditional Perl golf rules (#! line not counted, except for the - and switches if any).

$ echo 79 0.15 | perl test.pl
[|||||||||||                          15%                                     ]
$ echo 25 0.76 | perl test.pl
[||||||||||76%|||||     ]
$ echo 39 -0.12 | perl test.pl
[                -12%                 ]
$ echo 25 7.6 | perl test.pl
[|||||||||760%||||||||||]

PHP 84 x 0.75 = 63

Edit: A less 'pretty' version, but it should be valid according to the rules:

[<?=str_pad(!fscanf(STDIN,~Ú›Ú™,$a,$b),$a*min(1,$b),~ß)|str_pad(100*$b.~Ú,$a,_,2)?>]

Output:

$ echo 79 0.15 | php progress-bar.php
[⌂⌂⌂⌂⌂⌂⌂⌂⌂⌂⌂___________________________15%______________________________________]
$ echo 25 0.76 | php progress-bar.php
[⌂⌂⌂⌂⌂⌂⌂⌂⌂⌂⌂76%⌂⌂⌂⌂⌂______]
$ echo 39 -0.12 | php progress-bar.php
[_________________-12%__________________]
$ echo 25 7.6 | php progress-bar.php
[⌂⌂⌂⌂⌂⌂⌂⌂⌂⌂760%⌂⌂⌂⌂⌂⌂⌂⌂⌂⌂⌂]

Original (98 x 0.75 = 73.5)

[<?=strtr(str_pad(!fscanf(STDIN,~Ú›Ú™,$a,$b),$a*min(1,$b),~ß)|str_pad(100*$b.~Ú,$a,~ü,2),~ü,~ß)?>]

Output:

$ echo 79 0.15 | php progress-bar.php
[###########                           15%                                      ]
$ echo 25 0.76 | php progress-bar.php
[###########76%#####      ]
$ echo 39 -0.12 | php progress-bar.php
[                 -12%                  ]
$ echo 25 7.6 | php progress-bar.php
[##########760%###########]