To get alphabetics, use the POSIX IsAlpha class, also it is usually easier and more maintainable to make loops into functional (based on key logic) rather than technical blocks (e.g. based on fh open state instead), for example:

use strict;
use warnings;
use ... etc.;

my $filename = 'filename';
my $msgfront = 'MESSAGE FROM SAT: ';
my $path = $ENV{ PATHNAME } . "/$filename";
open my $ifh "<$path" or die $msgfront . "$!: $filename\n";

# 'group' loop
for ( $_ = <$ifh>; $_ = <$ifh> and /:IsAlpha:/; ) {
    chop;
    my ( $group, @address ) = ( $_, () );

    # 'address' loop
    for ( $_ = <$ifh> ; $_ = <$ifh> and !/:IsAlpha:/ ) {
        chop and push @address, $_;
    }

    # process $group with its @address -es here

}

close $ifh;
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