Using DateTime:

$ perl -l
use strict;
use warnings;
use DateTime;
my $str = '2007_12_13_04_49_15';
$str =~ /^(\d+)_(\d+)_(\d+)_(\d+)_(\d+)/;
my $dt = DateTime->new( 
  year   => $1,
  month  => $2,
  day    => $3,
  hour   => $4,
  minute => $5
);
print $dt->ymd . ' ' .  $dt->hms;
$dt->add( days => -1 );
print $dt->ymd . ' ' .  $dt->hms;
__END__
2007-12-13 04:49:00
2007-12-12 04:49:00
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