use warnings;
use strict;

my $_sensor_type =  "I2600A8600C9800";

if($_sensor_type =~ /[a-z]+([0-9]+)/ig)
{
  print "$1 OK\n"; #prints Ok
}

while ($_sensor_type =~ /[a-z]+([0-9]+)/ig)
{
  print "$1 OK with global loop\n";
}

__END__
prints:
2600 OK
8600 OK with global loop
9800 OK with global loop

So happened to the first "I2600", answer is the
/g in first match! Don't use /g (global unless you
mean to use this in a list context)

my $_sensor_type =  "I2600A8600C9800";

if($_sensor_type =~ /[a-z]+([0-9]+)/i)
{
  print "$1 OK\n"; #prints Ok
}

while ($_sensor_type =~ /[a-z]+([0-9]+)/ig)
{
  print "$1 OK with global loop\n";
}

__END__
now you get:
2600 OK
2600 OK with global loop
8600 OK with global loop
9800 OK with global loop