Nit: "Boolean context" is not relevant. Scalar vs list is what makes the difference.

print's operands are evaluated in both list context and string context. Is there an operator that has operands in both list context and boolean context? It would be possible to write such a sub.

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
[download]

Yet some more code about this... the /g operation generates a list, below I assign all regex matches to a list called @types.

use warnings;
use strict;

my $_sensor_type =  'I2600*&^_A8699()^%#@C9800&*%A96';

my @types = $_sensor_type =~ /[a-z]+([0-9]+)/ig;

#parens aren't needed here but to make list clear,
#could have been:
#my @types = ($_sensor_type =~ /[a-z]+([0-9]+)/ig);

print join("\n",@types),"\n";
__END__
## prints ###
2600
8699
9800
96
[download]

Another question that comes up all the time is about this $1 stuff and what a pain it is. So how to you avoid having to use $1? Use a list slice to assign $1 to a scalar without having to go via $1!

my $_sensor_type =  'I2600*&^_A8699()^%#@C9800&*%A96';
my $type = ($_sensor_type =~ /[a-z]+([0-9]+)/i)[0];
print "$type\n"; #prints 2600

#so what happens if the match and consequent slice fails?
$type ||= "some default";
#or check for undef
[download]

There are lots of cases where this trick is handy. Some types of split scenarios can produce undef values and this special Perl operator ||= can override that to a value, like even "".

Thank you for pointing out the significance of the /g identifier. I have removed it and my script is working as expected.